For those who don't know, Perixx (computer-peripheral vendor) sells replacement balls in the most common sizes for consumer-grade trackballs:

34 mm (Peripro-303), compatible with Perixx' own Perimice-517/717, Logitech thumb-trackball line—TrackMan Wheel, M570 and MX Ergo—and Elecom EX-G/DEFT; available in red/green and glossy/matte (I've heard that the glossy version messes with tracking in some trackballs)